1. Field of Invention
This invention relates generally to telephone line test systems and more specifically to provisioning a line test system.
2. Discussion of Related Art
Line test systems are widely used by telephone operating companies to increase the quality and reduce the cost of telephone service. 4TEL™ and Celerity® telephone line test systems provided by Teradyne, Inc of Deerfield, Ill., are examples of such systems.
A line test system typically consists of a test system controller and multiple test heads that may each be connected to a group of subscriber lines. To test a subscriber line, a test head takes measurements of multiple electrical characteristics of that subscriber line. These measurements are analyzed in the test system controller, which generates test results.
The type of analysis and the type of results generated depend on the reasons for conducting the test. In some instances, the test is performed in response to a report from a particular subscriber of a problem with their telephone service. In these instances, the test results may be used to identify a physical defect on a subscriber line that needs to be repaired. Additionally, the test results may indicate a location within the telephone network of a defect. Defect location information is useful to the telephone operating company because it aids in dispatching a repair person to repair the subscriber line.
Other testing scenarios arise because telephone operating companies want to use subscriber lines to carry high speed data signals instead of or in addition to voice information. However, high speed data signals have different characteristics than voice signals and a subscriber line that is capable of carrying a voice signal may not reliably carry a high speed data signal. Line test results may be used to predict whether a particular subscriber line will be able to carry high speed data signals or to predict the data rate that a particular subscriber line will support.
Many of the test results generated using a line test system depend on an estimate of the length of a subscriber line under test. For example, the length of a subscriber line may be used in identifying the location of a physical defect to better direct a repair person to the site where repairs are needed. An estimate of the length of a line may also be used in predicting the ability of the line to carry high speed data signals. Because long lines may attenuate high speed data signals too much for accurate transmission, the length of the line often is a good indicator of the types of signals that may be transmitted on the line.
To enable the test head to be switched to one subscriber line, in a typical test system the test head is installed in a central office, which contains a switch connected to multiple subscriber lines. To run a test, the test head is connected to a selected subscriber line through the switch. A drawback of such a configuration is that the path from the test head to the subscriber line under test passes through the switch, a distribution unit and cabling within the central office. Much of that path between the test head and the distribution unit is not used to carry signals when the telephone network is in normal operating. Therefore, it is often preferable to know the length of the line from the distribution point without the impact of the portion of the path between the test head and the distribution point.
To increase the accuracy of line length estimates, it is known to estimate the effects of a path between the test head and the distribution unit. Values representing this path can be subtracted from measurements made on a line under test to produce line measurements that more accurately reflect the subscriber line as it is used. The amount by which the path through the central office impacts measurements of the line is sometimes reflected as “CO limits.” When installing, or provisioning, a test system, the CO limits are sometimes determined and stored in the test system controller. The CO limits can then be subtracted from measurements made with the test head so that an estimated line length more accurately reflects the actual length of a subscriber line.
As a test system is provisioned, it is known to make measurements of equipment and cabling in the central office to determine the effect of that equipment and cabling on subscriber line measurements made with a test head installed at the central office. This process involves making measurements on multiple lines. Some of these measurements are two-ended, meaning that measurements are made with a device at one end of the line while either a signal or operating state on the line is altered in another location. For example, a fault condition may be introduced on the line and measurements made while the line is in this fault state. Two-ended measurements require test equipment in at least two locations, and frequently require people in multiple locations.